The effects of Fe, Mg, and Pt-doping on the improvement of Ni stabilized on Al2O3-CeO3 catalysts for methane dry reforming

Herein, the promotional effects of Mg, Fe, and Pt on Ni-based catalysts supported on Al2O3-CeO2 (Ni/Al2O3-CeO2) were investigated in the dry reforming of methane (DRM) reaction. The interaction of a suitable amount of MgO and FeO with Ce2O3 stabilized in the catalysts was demonstrated by the temperature-programmed desorption of CO2 (CO2-TPD). Ce2O3 has a high basicity for adsorbing CO2, generating a monoclinic Ce2O2CO3 species in the DRM reaction. Surface oxygen ions were also produced by adding MgO and FeO, as demonstrated by the temperature-programmed reduction of H2 (H2-TPR). Monoclinic Ce2O2CO3 and surface oxygen may both be used to oxidize and remove the carbon that was deposited, maintaining the high activity and stability of the metal Ni and Pt catalysts. The high dispersion and synergistic interactions between the platinum and oxide phases, which are associated with the decrease in reduction temperature and the rise in the number of basic sites, are responsible for the increased activity of Pt with M–Ni/Al2O3-CeO2 catalysts. The co-doped Ni/Al2O3-CeO2 catalysts with Mg and Fe significantly enhanced the activity (more than 80% methane and 84% CO2 conversion), the selectivity toward syngas (∼90%), and maintained the H2/CO ratio at about 0.97 at 700 °C.


Introduction
][3] DRM is an endothermic reaction and is usually conducted at a very high operation temperature (>800 °C) to ensure high methane conversion and minimize carbon deposition thermodynamically. 4,5he majority of the catalysts investigated for DRM are generally made up of group VIII transition metals, such as Ni due to their high activity. 6,7][9][10] In particular Pt, Rh, and Ru are highly active towards DRM, which enhances the stability against coke deposition as compared to the other non-promoted nickelbased catalysts. 11,12The catalyst performance is dependent on the Ni/metal ratio and the nature of the support. 6,13,14Both the promoters and support play important roles in metal electron transfer, cluster stabilization, and reducibility. 15In particular, it has been shown that Ni catalysts are highly reducible in the presence of noble metals, which enables both methane combustion and reforming to occur simultaneously, thereby resulting in higher energy efficiency and improved catalytic activity. 4Although noble metals are much more resistant to carbon deposition than other metal-based catalysts, they are generally uneconomical.Developing bimetallic catalysts by combining nickel with other metals is an alternative route to obtaining highly coke-resistant Ni-based catalysts for the DRM reaction. 16Several studies have been dedicated to improving the Ni-based CeO 2 -Al 2 O 3 performance and stability through the addition of second metal promoters, such as Co, Pd, and Pt.It has been conrmed that adding a trace of transition metals can modify Ni surface properties by promoting the reducibility of Ni and thus increasing the number of active sites to achieve better catalytic performance. 9,17[20] a Department of Chemical & Biochemical Engineering, Missouri University of Science and Technology, 1101 N. State Street, Rolla, Missouri 65409, USA.E-mail: ajd5d@ mst.edu; abbasajd5d@gmail.comb Midland Reneries Company MRC, AL Daura Renery Company, Services Energy Board, Baghdad, Iraq Developing bi/trimetallic catalysts is a viable and critical method for developing highly coke-resistant Ni-based DRM catalysts, and there has been tremendous research interest in this regard in recent years.Combining nickel with other metals can easily change its surface properties, resulting in improved catalytic activity; this phenomenon is known as the synergistic impact. 21Noble metal catalysts such as Pt, Ru, and Rh exhibit good performance and selectivity for DRM reactions, even though noble metals have a high stability to carbon agglomeration at high temperatures. 6However, due to their high cost and scarcity, they are not economically competitive with other transition metal-based materials.Elsayed et al. 22 discovered that supported bimetallic catalysts have good activity and stable DRM reaction capabilities.To assess the stability, platinum (0.2-2 wt%), nickel (8 wt%), and magnesium (8 wt%) were immobilized onto a ceria-zirconia support.Precipitation was used to create ceria-zirconia (0.6 : 0.4) solid solutions, and the metals were loaded using the incipient wetness method.The combination of Pt with NiMg/(Ce,Zr)O 2 catalysts improved lowtemperature dry reforming activity when compared to the control catalysts without Ni, Mg, and Pt.Among the various supports investigated, Al 2 O 3 and HZSM-5 possessed greater surface areas, which enhanced the Ni particle dispersion; nevertheless, their inherent acidity and the greater interactions between the metal and support led to the rapid deactivation of the catalyst due to severe coke deposition.With the excellent thermal stability of CeO 2 and the high specic surface area of Al 2 O 3 , which promotes the binding of tiny metal nanoparticles while also promoting CO 2 binding in the catalytic processes by generating exible carbonate-like species, 23 a combined framework of these two materials was proposed and showed enhanced catalytic performance. 7,24,25In particular, cerium oxide has been widely investigated by many researchers as a promoter and support for nickel-based catalysts due to its unique redox properties (Ce 4+ /Ce 3+ ) and remarkable oxygen storage capacities for the DRM reaction. 26Fang et al. 27 investigated the impact of the promotors (Ce, Zr, and Al) employed to promote Y 2 O 3 as supports for Ni/NiO to obtain more efficient catalysts for the DRM process.The XRD and Raman data showed that all three cations were doped into the Y 2 O 3 lattice to form a solid solution structure, resulting in supports with decreased crystallinity and enhanced surface areas.As a result, all the changed catalysts had increased reaction performance.The Ni-support interaction of the modied catalysts was improved as compared to the unmodied catalyst, resulting in improved Ni dispersion.Furthermore, the modied catalysts had increased alkalinity, which is advantageous for activating CO  Aluminum hydroxide was precipitated from an aqueous Al(NO 3 ) 3 solution using ammonia as the precipitating agent.At a temperature of 80 °C, the precipitation process was carried out by adding ammonia to the nitrate solution until the pH of the solution changed from acidic to basic (pH = 9-10).The Al(OH) 3 precipitate was aged for 24 h, then ltered and rinsed with deionized water until the solution's pH was 7. The precipitate was obtained, which was then dried at 100 °C for 1 h and calcined at 550 °C in an oxygen stream for 4 h.

Experimental
Cerium(III) nitrate hexahydrate was used to create the CeO 2 -Al 2 O 3 support.The aqueous nitrate solution was added dropwise to the previously obtained alumina support, then, the resulting solution was aged for 24 h.Aer the solvent had evaporated, the solid residue was dried at 100 °C for 1 h before being calcined in air for 5 h at 550 °C.CeO 2 was loaded to a 50% level.
2.1.3Synthesis of monometallic systems.The monometallic Ni catalyst was prepared using the incipient wetness impregnation method.An aqueous solution of nickel(II) nitrate was used to deposit a nickel phase onto CeO 2 -Al 2 O 3 , which was then impregnated for 24 h.The catalyst was produced, dried at 100 °C, and then calcined at 550 °C for 5 h aer the solvent had evaporated.The catalyst was obtained with a nominal metal concentration of 4%.
2.1.4Synthesis of bimetallic systems.The bimetallic catalyst Fe-Ni was obtained by subsequent impregnation using an aqueous Fe(NO 3 ) 3 solution.The impregnation process was the same as previously stated.The catalyst was obtained with a nominal Fe concentration of 2%.
2.1.5Synthesis of trimetallic systems.The trimetallic catalyst Mg-Fe-Ni was obtained by subsequent impregnation using an aqueous Mg(NO 3 ) 2 solution.The impregnation process was the same as previously stated.The catalyst was obtained with a nominal Fe concentration of 2%.The aqueous H 2 PtCl 6 $6H 2 O solution was impregnated into (Mg-Fe-Ni and Fe-Ni) catalysts.The impregnation procedure was similar to that reported before.The nominal Pt content in the obtained catalysts was 0.0005%.

Instrumental measurements
X-ray diffraction (XRD) patterns of the catalysts were obtained by a diffractometer using a PANalytical instrument operating at 30 kV and 15 mA.The XRD pattern was evaluated at a step size of 0.026°from 2q = 5°to 90°and a rate of 2°min −1 .N 2 physisorption isotherm measurements were carried out in a Micromeritics 3Flex surface characterization analyzer at 77 K. Textural properties such as surface area, total pore volume, micropore volume, and average pore width were determined using Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), and t-plot methods, respectively.Prior to the measurements, samples were degassed at 250 °C for 6 h using a Smart VacPrep.H 2 -TPR measurements were carried out in a U-shaped quartz cell using a 5% vol H 2 /He gas with a ow rate of 50 cm 3 min −1 at a heating rate of 10 °C min −1 up to 900 °C by using a Micromeritics 3Flex analyzer.Hydrogen consumption was followed by on-line mass spectroscopy (BELMass) and quantitative analysis was done by comparison of the reduction signal with the hydrogen consumption of a CuO reference.The temperature-programmed desorption of CO 2 (CO 2 -TPD) was performed on the same Micromeritics 3Flex analyzer.Prior to adsorption measurement, all samples were initially reduced at a temperature of 200 °C in a 5% H 2 in He gas mixture and held at the reduction temperature for 1 h, then cooled down to 50 °C under He.Aer the temperature was stabilized, the sample was exposed to 10% CO 2 in He for 30 min.To remove physically bound CO 2 from the surface, a ow of He (50 cm 3 min −1 ) for 30 min at 50 °C was used.The desorption of CO 2 was measured from 50 to 600 °C at a constant heating rate of 10 °C min −1 .To determine the nature of surface acid sites, Fourier-transform infrared spectroscopy (FTIR) of pyridine, using a Bruker Tensor spectrophotometer, was employed to determine the types of acid sites present in the samples.All samples were activated at 450 °C for 4 h to release the moisture before the adsorption of pyridine and cooled down to 60 °C for pyridine adsorption until saturation.All the measured spectra were recalculated to a "normalized" wafer of 10 mg.For the quantitative characterization of acid sites, the bands at 1450 and 1550 cm −1 were considered to correspond to Lewis and Brønsted sites, respectively.Furthermore, inductively coupled plasma mass spectrometry (ICP-MS) analyses were used to obtain the chemical composition of the surfaces and bulk before reaction.

Catalytic tests
Catalyst tests were carried out in a stainless-steel packed-bed reactor with an internal diameter of 10 mm and a length of 300 mm, as depicted in Scheme 1.The feed gas consisted of either pure CH 4 or 50% CH 4 /CO 2 and its ow rate was controlled by a digital mass ow controller (MFC, Brocks Instrument) towards the reaction zones.For each run, about 300 mg of the sample (particle size 0.5 mm) was diluted with sand (particle size 0.5 mm) at the ratio of 1 : 6 and placed in the center of the reactor with quartz wool at both ends.Prior to the reaction, the catalyst was activated in situ at 500 °C in H 2 ow for 1 h.Each catalyst was evaluated within a temperature range of 550-700 °C at a constant weight hourly space velocity (WHSV) of 12 000 mL g −1 cat h −1 .The reaction temperature was controlled by embedding a type-K thermocouple inside the catalyst center.The reactions were carried out isothermally for 10 h time-onstream.The reaction products were analyzed online every 30 min with a gas chromatograph (SRI 8610C) equipped with a ame ionized detector (GC-FID) and thermal conductivity detector (TCD) for H 2 , CO 2 , CO, H 2 O, and hydrocarbons.The effluent line of the reactor to the GC injector was kept at 110 °C to avoid the potential condensation of the hydrocarbons.From Table 1, the Ni, Mg, Fe, and Pt percentages were found to be very close to the theoretical values using ICP elemental analysis, which could be due to the incomplete precipitation of the nickel, magnesium, iron, and platinum metal precursors used during the co-impregnation process.This could be due to the presence of a small amount of oxide and also the successful incorporation of these metals into the Ni-based Al 2 O 3 -CeO 2 structure. 29As listed in Table 1, elemental analysis conrmed the presence of these metals.The average crystalline size of nickel was calculated by using the Debye-Scherrer equation (Table 1).As a function of the metal oxide(s) content, the nickel crystal size decreased from 36 to 26 nm.As a result, the crystallite size also produced oxygen vacancies, in addition to the production of oxygen vacancies mediated by dopants. 30This phenomenon could be associated with the effects of the metal oxides and nickel that remained on the surface of the sample and inhibited the growth of Ni crystals.This was conrmed by the ICP elemental analysis results shown in Table 1.

Results and discussion
The H 2 -TPR proles of the reduced Ni-based Al 2 O 3 -CeO 2 composite catalysts showed four well-dened reduction peaks in the range of 550-850 °C (Fig. 2).These reduction peaks correspond to NiO incorporated within the structure of Al 2 O 3 -CeO 2 . 31The broad peak at 492-793 °C is associated with the bulk reduction of Ce 4+ to Ce 3+ . 32The process of NiO reduction is thought to be a complex solid-gas reaction driven by temperature, reducing gas concentration, and the presence of additives (Mg, Fe, and Pt). 33This is consistent with the ndings of Manukyan et al. 34 The decrease began with the initial nucleation of Ni in the temperature range from (265 to 545 °C).During this phase, Ni nuclei appeared on the outer surface of NiO particles.This gradual process eventually resulted in the formation of small channels, which began to propagate and initiate the second step of the reduction.A dominant peak was formed between 565 and 900 °C, indicating the second stage of reduction on the majority of the NiO and metal oxide(s) particles.The This implies that the platinum interaction with the support also has a signicant effect on increasing the reducibility of the support. 38Platinum helps reduce the oxide phases through its ability to facilitate dissociative hydrogen adsorption.Hydrogen has been observed to adsorb and dissociate on the surface of the platinum, whereby it spills over to the entire surface of the support. 7,17On the metallic surface, hydrogen molecules dissociate into hydrogen atoms that diffuse to the Ni 2+ and can react with NiO, resulting in the uptake of the hydrogen.Nevertheless, the rate of reduction of NiO depends not only on its chemical nature but also on the nucleation process by which metallic nuclei are generated.The better interaction between the nickel particles and MO x -doped ceria supports could be associated with an anchoring effect inside the mesoporous structure of the support, as revealed by the textural characterization results (Fig. 5 and Table 3).It has been demonstrated that metal nanoparticles conned in mesoporous CeO 2 enhance the metal support interaction and, therefore, the catalytic activity. 39Specically, a stronger Nibased Al 2 O 3 -CeO 2 interaction inhibits the Ni particle agglomeration and coke deposition, which leads to better stability of the catalysts in the DRM reaction.
The number of surface alkaline centers also has an important inuence on the DRM process.According to prior research, surface alkaline sites can adsorb and activate CO 2 molecules, resulting in reactive intermediates and surface oxygen species. 40he adsorbed oxygen species are active in removing the initially produced carbon deposition over time, which is benecial for the DRM process.As a result, CO 2 -TPD was employed to characterize the surface alkalinity of the catalysts.The CO 2 -TPD results are shown in Fig. 3 and changes in the weak, moderate and strongly basic sites are presented in Table 2.The CO 2 -TPD prole of the samples shows weakly basic sites between 60 and 200 °C, moderately basic sites between 200 and 400 °C, and strong sites between 400 and 800 °C, respectively. 27The total amount of CO 2 was estimated from the integration of the CO 2 -TPD peak area desorbed, which obviously increased with the addition of metal oxides as compared to Ni-based Al 2 O 3 -CeO 2 .Typically, it was reported that the peak below 200 °C was attributed to the desorption of CO 2 on the weakly alkaline sites.Due to its easy desorption, this part of adsorbed CO 2 could have a limited contribution to the reaction.The CO 2 adsorbed on strongly basic sites was desorbed at high temperatures. 27As a result, the CO 2 desorption peak area improved signicantly at higher temperatures, indicating that the amount of adsorbed/ desorbed CO 2 was increased at 550 °C, which is in the range of the reaction temperature and favorable for improving the reactivity.
Koo et al. 41 reported that besides weak basic sites, moderate basic sites, and strong basic sites are favorable for depressing the coke formation in MgO-promoted Ni catalysts.In our study,  .This is attributed to the increased basicity of the catalysts, which in turn increased the rate of activation of mildly acidic CO 2 and hence assisted in the oxidation of surface carbon and increased the catalyst resistance to deactivation. 7Compared to M-Ni tri/bimetallic catalysts, Pt-modied M-Ni tri/bimetallic catalysts greatly increased the contribution of CO 2 deoxidation, and the excellent Pt deposition over M-Ni tri/bimetallic catalysts can be used to explain this, allowing oxygen to diffuse from the metal or support to the Pt o in a process known as "reverse oxygen spillover".This produces PtO and an increase in the concentration of the metal oxide(s) ions, which are the active sites for CO 2 reduction. 44The PtO was detected by XRD (Fig. 1), which was proven by previous research. 45Eventually, the catalyst was provided with active oxygen species that suppressed carbon deposition, followed by catalyst deactivation.Three desorption peaks centered around 395, 489, and 569 °C were also observed for all the catalysts despite the latter two peaks overlapping.The strength of the overlapped peaks was closely related to the addition of the metal oxides.These three peaks might be related to the strongly chemisorbed CO 2 .To meticulously investigate these TPD proles, the Lorentz mathematical model was used to resolve the overlapped desorption peaks.For example, as shown in Fig. 3b, four distinct desorption peaks centered at 65, 268, 328, 386, 492, 572, and 707 °C were observed over the Pt/MgFe/Ni/Al 2 O 3 -CeO 2 catalyst.This implies that more than one type of basic centers with different intensities existed in the mesoporous framework of the Pt/ MgFe/Ni/Al 2 O 3 -CeO 2 composite catalyst.Overall, the categories of the basic centers for Ni-based Al 2 O 3 -CeO 2 catalysts were abundant due to their own structural features as well as the promotion of the metal oxides and this is in agreement with previously reported data. 46he Brønsted and Lewis sites were found via the ex situ FTIR spectra of pyridine adsorption using a Bruker Tensor spectrophotometer.The FTIR spectra (Fig. 4 and Table 2) showed the impact of the addition of metal oxides (M = Pt, Mg, and Fe) on the Ni-based Al 2 O 3 -CeO 2 surface acid site.One milliliter of dried pyridine was adsorbed on 30 mg of the catalyst for 14 h.The sample was dried at 120 °C for 1 h to release the loosely adsorbed pyridine from the surface of the catalyst.From that dried sample, 10 mg was taken and, to ensure homogeneity, mixed thoroughly with 200 mg of dry KBr.The mixture was then pelletized using hydraulic pressure and the FTIR spectra of the  pellets were then obtained.According to Fig. 4, an absorption band at 1440 cm −1 or 1434 cm −1 , which corresponds to the pyridine coordinated on Lewis acid sites on all of these catalysts, was identied in all catalysts, indicating the interaction between the pyridine and metal oxides. 47The bands on these catalysts at 1617 cm −1 and 1596 cm −1 were attributed to pyridinium cations generated aer pyridine adsorption on Brønsted acid sites and the interactions between the adsorbed pyridine and metal oxides, respectively. 48,49 respectively, suggesting that the addition of metal promoters reduced the total surface area.All the investigated metals inuenced the textural properties of the Ni-based Al 2 O 3 -CeO 2 catalyst but the extent was varied from metal to metal.This might result from the added metals physically blocking the support pores. 53The results suggest that the doped metals entered the pores of the Al 2 O 3 -CeO 2 during the doping process, thereby affecting the mesoporosity and pore volume of the support.5 and 6, the activity and stability of the catalysts, as well as the H 2 /CO product ratio were measured at 550-700 °C for 600 min time-on-stream.For all catalysts, the CO 2 conversion was higher than that of CH 4 and the conversions of both CO 2 and CH 4 were enhanced with increasing reaction temperature, indicating the occurrence of side reactions such as the reverse water gas shi reaction (CO 2 + H 2 / CO + H 2 O). 7,29In the absence of CO 2 feeding (pure CH 4 ), a signicant amount of CO was formed over different catalysts due to the decomposition of methane (CH 4 / C + H 2 ) and a further oxidative regeneration (C + O 2 / CO x ) process. 54The H 2 /CO ratios were also increased as the reaction temperature increased because the RWGS reaction would gradually be prevented at elevated temperatures. 46The presence of CO 2 promoted CH 4 conversion (Fig. 6, Tables 5 and 6), catalyst stability and H 2 /CO ratio as compared to the catalysts in the absence of CO 2 (Fig. S1 †) at all reaction temperatures.

Catalytic performance in DRM
The catalytic stability of the catalysts was investigated over 10 h at varied temperatures (550, 650, and 700 °C) and constant mass of catalyst.Fig. S1 † depicts the CH 4 , and CO 2 conversions, as well as the H 2 /CO molar ratio as a function of time on-stream (TOS).These results conrmed the better performance of the metal oxide(s)-doped Ni/Al 2 O 3 -CeO 2 catalyst as compared to the Ni/Al 2 O 3 -CeO 2 catalyst.The results revealed that all catalysts had good activity and stability during 10 h on stream (TOS).It may be concluded that the carbon deposition on the surface of the catalyst was minimal, resulting in the stable conversion and   H 2 /CO ratio was lower than the stoichiometric value for DRM (1 mol mol  5 and 6).Notably, the formation of CO was higher for all Fe-doped catalysts, thereby precluding the accumulated carbon deposition on nearby Ni atoms and enhancing the catalytic activity for non-oxidative methane dehydrogenation and DRM.The redox mechanism is well-dened as the Mars-van Krevelen (MvK), which consists of two reactions: the rst reaction is a reduction of the catalyst via hydrocarbon (methane), and the second is the re-oxidization reaction of the catalyst. 56Doping the FeNi/Al 2 O 3 -CeO 2 catalyst with 0.005 wt% Pt further improved the catalyst stability and maintained the H 2 /CO ratio, probably by the initial dissociation of methane (CH 4 / CH 3 + H) as shown in Fig. 6A-C, Tables 5  and 6.Previous studies 11,12 also conrmed that the presence of Pt sites can also initiate the reduction of NiO by the rapid dissociation of H 2 and then the migration of atomic H to the NiO surface by the phenomenon of hydrogen spillover, which produces a higher mobility of hydrogen on the support surface, facilitating the access to Ni particles.The Pt/FeNi/Al 2 O 3 -CeO 2 catalyst showed more resistance to carbon formation than Ni/ Al 2 O 3 -CeO 2 and FeNi/Al 2 O 3 -CeO 2 catalysts (Fig. 6A-C, Tables 5  and 6).As noted in Table 5, these ndings have been conrmed by calculating the deactivation factor and are consistent with literature reports. 7A previous study by Pawelec et al. 57 demonstrated that adding 0.005%Pt to the Ni catalyst leads to the generation of nanosized NiO particles, which can be readily reduced.Based on their ndings, the authors ascribed the enhancement in the performance and coke resistance over the Pt-Ni catalysts to the increase in the nickel metallic dispersion caused by the intimate contact between nickel and platinum.
The effect of Mg doping on the catalytic performance of the FeNi/Al 2 O 3 -CeO 2 catalyst was also examined, as illustrated in Fig. 6A-C, Tables 5 and 6  When compared to monometallic catalysts, it reduces the activation energy of CH 4 dissociation and CO 2 activation.Furthermore, it increases the responsiveness of surface oxygen species, which improves carbon species suppression.The results (Table 7) show that Pt/M-Ni has lower carbon deposition than Ni due to (i) a higher energy barrier for the decomposition of the CH * x species, which led to C* (CH * , plausible catalytic mechanism) and (ii) lower energy barriers for the oxidation of CH * x species (CH , plausible catalytic mechanism, where CH x O* is an intermediate for CO*) and C* species (C* + O* = CO* + *).As a result, the carbon concentration on the surface of Pt/M-Ni was lowered.Pt/M-Ni also weakens hydrogen dissociation, lowering the RWGS and increasing H 2 generation.To better understand the inuence of metal promoters on the activities of the M-Ni/Al 2 O 3 -CeO 2 catalysts for the DRM, the turnover frequencies (TOFs), which reect the intrinsic activity of the active sites, were calculated based on the Ni crystallites from XRD and the initial CH 4 conversions (X CH 4 ) at 550-700 °C, and the results are listed in Table 5.For all catalysts, the TOF was improved by increasing the reaction temperature, and also by secondary and tertiary metal doping, consistent with the variation in CH 4 conversions.These observations are in agreement with the conversion and TPR results, which revealed the best catalytic activity and the highest reducibility for Ni-based catalyst-doped metal oxides.This could be ascribed to the high interaction and dispersion of metal dopants on the catalyst surface in accordance with XRD results (Table 1).The M-Ni tri/bimetallic surface modied with Pt had a lower binding energy for H* than the monometallic Nibased Al 2 O 3 -CeO 3 catalyst.This demonstrates that on Pt-modied M-Ni tri/bimetallic catalysts, the surface oxidation step leading to CO production (potentially CH* + O*) became the kinetically signicant phase.The electrical modication by generating the Pt-modied M-Ni tri/bimetallic surface can be attributed to the catalyst composition's signicant activation energy dependence. 62The trend of shiing the CO stretching frequency as a result of the charge transfer between Pt and Ni ts well with the tendency of variations in the methane activation energy.Additionally, the methane activation energy tends to be similar to the O* adsorption energy, but the CO 2 activation energy tends to be similar to the C* adsorption energy with catalyst composition.This emphasizes the relationship between the heat of C* or O* adsorption and the apparent activation energy.The activation energy of CO 2 increases as the C* adsorption energy increases, while the activation energy of CH 4 increases as the O* adsorption energy increases.This agrees with earlier literature and emphasizes the relationship between increases as the O* adsorption energy increases.This agrees with previous literature. 63The results show that a minimal addition of Pt could signicantly reduce the energy required to  .This is consistent with the ndings of CO 2 -TPD (Fig. 3).Enhancing the catalyst's resistance to the generation of coke and extending its useful life during the reaction process is advantageous.According to H 2 -TPR investigations (Fig. 2), the presence of Pt in the Ni particles causes easily reducible NiO particles, thereby decreasing the reduction temperature because of the strong contact between Ni and Pt.As a result, the Ni-Pt matrix (alloy phase) generated with CO 2 dissociation on the metal surface is easier to remove from the surface and subsurface oxygen.Similarly, the Ni-based Al 2 O 3 -CeO 3 catalyst's reducibility increased and the temperature at which the NiO species were reduced dropped aer the addition of metal oxides (M = Mg and Fe).This is consistent with the ndings of H 2 -TPR (Fig. 2).The estimated TOF numbers are consistent with the literature ndings that demonstrated higher methane conversion and syngas yield at higher TOF numbers. 64n general, the coke generated by methane decomposition (step (6)

Plausible catalytic mechanism
Metal oxide(s) as promoters are crucial in the development of extremely effective DRM catalysts.All the data show that the additions had a positive effect on the surface basicity, redox characteristics, and dispersion of Ni particles, and knowing these favorable qualities can help to clarify the catalytic mechanism.The activation of CH 4 and CO 2 is commonly regarded as a critical step. 70As illustrated in Fig. 7, CH 4 is activated over the Ni sites, and CO 2 can be adsorbed and activated on the support surface, and the metal oxide and support interface.As demonstrated in CO 2 -TPD proles, increasing the basicity of catalysts can increase the rate of CO 2 activation, which has a major impact on catalytic performance (Fig. 3 CH* + * 4 C* + H* Gathering steps (2)-( 5) gives step (6): CO * 2 þ *4CO * 2 ; CO 2 adsorptionðmetal oxideðsÞ or metal-support interfaceÞ (7) (*) and O LT represent unoccupied active sites, and lattice oxygen on the metal oxide surface, respectively.According to the above analysis, CH 4 dissociative adsorption occurs on the Ni metal surface (steps (2)-( 5)), while CO 2 is adsorbed on the surface of the catalysts in the form of the carbonate species (step (7)).As the carbonate species encounter the methane pyrolysis products, they degrade fast into formate species, which subsequently decompose further into CO (step (9) and step (10)).CO 2 reacts with H* generated by methane dehydrogenation to yield COOH* or HCOO* (step (11)).COOH* is subsequently decomposed into COH* and O* (step (12)) or CO* and OH* (step (13)) before COH* dehydrogenation to produce CO* and H* (step ( 14)).Adsorbed CH x is oxidized via OH* groups to create CH x OH* (step (15)), which is then dissociated into CO* and H* (step ( 16)).The lattice oxygen, produced by the dissociation of CO 2 , and the facile movement of oxygen could be Finally, H * 2 and CO* are desorbed on the catalyst to form H 2 (g) and CO(g) (step ( 22) and ( 23)).Fan et al. 72 postulated a different reaction mechanism for catalysts with basic supports, such as CeO 2 , in which CO 2 activation occurs on the surface of the support rather than the metal active site.In this mechanism, CO 2 is adsorbed in the vicinity of the metal particles on the catalyst support, CO 2 (g) = CO 2 (support), resulting in the carbonate species, CO 2 (support) + O 2− = CO 3 2− (support).The hydrogen is then used to reduce the carbonate (CO 3 2− (support) + 2H = HCO 2 − (support) + OH − ) to produce CO. 73According to the theory, CO 2 is adsorbed on promoters like Ce or CeO and dissociates into CO and O (step ( 5)).Following this, carbon from the decomposition of CH 4 is deposited on the catalyst's active sites where it reacts with the adsorbed oxygen on the promoter to generate CO (O* + C* / CO*).

Conclusion
2 and increasing activity.Most of the published studies reported improvement in catalytic behavior, however, little is known about the nature, structure and performance of Ni-M/ CeO 2 -Al 2 O 3 catalysts, where M = Fe, Fe-Mg, and Fe-Mg-Pt.The aim of this study is to improve the performance of Nibased CeO 2 -Al 2 O 3 composite catalysts by synthesizing and structurally characterizing a series of monometallic M-Ni (M = Fe), bimetallic M-Ni (M = Mg-Fe and Pt-Fe) and trimetallic M-Ni (M = Mg-Fe-Pt) supported on CeO 2 -Al 2 O 3 to suppress coke formation and maintain the H 2 /CO ratio.The role of the Pt co-promoter in composite catalysts was studied in the DRM reaction to distinguish the catalytic effects of Pt.Among all the synthesized M-Ni/CeO 2 -Al 2 O 3 catalysts, the Pt/MgFe/Ni/CeO 2 -Al 2 O 3 catalyst has shown the best performance by enhancing the CH 4 and CO 2 conversion and selectivity toward syngas, as well as extending the catalyst life.

3. 1 .
Characterization of the catalyst The powder X-ray diffraction patterns for the thermally calcined Ni-based Al 2 O 3 -CeO 2 composite catalysts with metal additives are shown in Fig. 1.The diffraction peaks observed at 2q = 28, 33, 48 and 57°indicate the presence of the cubic crystal structure of the CeO 2 support. 28All samples displayed diffraction peaks at 2q = 38, 45, 67°, which are attributed to the g-Al 2 O 3 support.Peaks were barely seen at 2q = 23.5, 35 and 60.5°, which can be assigned to the NiO (006), NiO (009) and NiO (110) phases, respectively.XRD signatures of MgO, Fe 2 O 3 , and Pt were not observed in the Ni-based Al 2 O 3 -CeO 2 composite catalysts.

Table 1 a
XRD and ICP analysis of the Ni-based Al 2 O 3 -CeO 2 composite catalysts Catalysts Metal loading a (wt%) 2 O 3 -CeO 2 48.1 48.0 -3.9 --36 FeNi/Al 2 O 3 -CeO 2 47.1 46.8 2.1 4.0 --32 Pt/FeNi/Al 2 O 3 -CeO 2 47.5 46.5 2.0 3.9 -0.005 26 MgFe/Ni/Al 2 O 3 -CeO 2 47.0 46.8 1.9 3.8 0.5 -33 Pt/MgFe/Ni/Al 2 O 3 -CeO 2 47.2 46.4 2.1 3.6 0.6 0.005 27 Determined by ICP analysis.b Estimated by the Debye-Scherrer equation for Ni (200) of XRD.Fig. 1 XRD patterns of Ni-based Al 2 O 3 -CeO 2 composite catalysts.small channels evolved into bulk NiO.Further Ni nucleation proceeded on the inner surface of these channels, resulting in the development of Ni nano-layers.The nano-layers then begin branching and interconnecting the network until the NiO phase was totally reduced to Ni. 32,34,35 The addition of Fe and Mg metals into the Ni-based Al 2 O 3 -CeO 2 catalyst modied the reduction process and caused the decay of the peak connected to unbound NiO, whereas the observation was made for the Fe 2 O 3 -doped systems.It should be noted that the presence of alkaline metal oxide(s) in the catalyst had a substantial impact on the reduction behavior of the NiO catalysts.In addition to the decrease in the temperature for the ceria reduction, promoted by the presence of the Fe 2 O 3 , the supplementary reduction of Fe 2 O 3 was observed as previously reported by Wimmers et al. 36 who studied the reduction of Fe 2 O 3 and proposed a reduction in two steps, Fe 2 O 3 / Fe 3 O 4 / Fe, with no formation of FeO.For the same oxide, other authors proposed a three-step reduction process, which considered FeO formation dealing with Fe 2 O 3 / Fe 3 O 4 at about 400 °C, Fe 3 O 4 / FeO at about 600 °C and nally, FeO / Fe at higher temperatures. 37Irrespective of the number of reduction steps of the Fe 2 O 3 , the separation of its reduction from that of the CeO 2 overlapping reduction zones is hard to achieve.However, the Nibased Al 2 O 3 -CeO 2 catalyst without any platinum content was reduced at a much higher temperature than catalysts with platinum.The addition of platinum to Ni particles causes easily reducible NiO particles, thereby decreasing the reduction temperature because of the strong contact between Ni and Pt.Therefore, the Pt/FeNi/Al 2 O 3 -CeO 2 and Pt/MgFe/Ni-based Al 2 O 3 -CeO 2 catalysts allowed the reduction in the selectivity for carbon and reached values closer to equilibrium at lower reaction temperatures.

Fig. 4
and Table 3 demonstrate that the addition of metal oxides resulted in a decrease in the creation of Lewis and Brønsted acid sites, respectively, as compared to the Ni-based Al 2 O 3 -CeO 2 catalyst.Conversely, it may be said that M-Ni-based Al 2 O 3 -CeO 2 has greater Lewis basicity than Ni/Al 2 O 3 -CeO 2 , which is the preferred material for CO 2 adsorption. 50As a result, the activity performance during the DRM is improved, and carbon deposition during formation is reduced.N 2 physisorption isotherms of the as-prepared samples are shown in Fig. 5 with the corresponding pore size distribution shown as inset gures.All isotherms exhibited a combination of the type IV isotherm with the H 4 type hysteresis loop, associated with capillary condensation, and indicated the formation of mesoporous structures in all the Ni-based Al 2 O 3 -CeO 2 composite catalysts.The BJH method was used to estimate the pore size distributions by using the adsorption branch.Metal oxides doped Ni-based Al 2 O 2 -CeO 2 catalysts displayed a hierarchically bi-modal porous structure with an apex of 3.8 nm (Fig. 5b) and the other between 34 and 38 nm, both of which fall within the mesoporous range; in contrast, Ni-based Al 2 O 2 -CeO 2 and Al 2 O 2 -CeO 2 only presented an apex.It has been established that the presence of the porous architecture in metal oxidesdoped Ni-based Al 2 O 2 -CeO 2 catalysts has a favorable inuence on the improvement of catalytic activity because the connected internal voids might potentially function as efficient transport channels. 30,52Table 4 summarizes the total surface area, micropore surface area, external surface area, mesopore volume pore size and diameter of Ni-based Al 2 O 3 -CeO 2 composite catalysts.For comparison, the Al 2 O 3 -CeO 2 powder was also measured.The surface areas of Al 2 O 3 -CeO 2 , Ni/Al 2 O 3 -CeO 2 and Pt/MgFe/Ni/Al 2 O 3 -CeO 2 were found to be 92, 86, and 76 cm 2 g −1 ,

Fig. 6 (
Fig. 6(A)-(C) present catalytic results of the DRM at 550-700 °C in the presence of pure CH 4 and a CH 4 /CO 2 mixture (50 : 50).As shown in Fig. S1 (ESI), † Tables5 and 6, the activity and stability of the catalysts, as well as the H 2 /CO product ratio were measured at 550-700 °C for 600 min time-on-stream.For all catalysts, the CO 2 conversion was higher than that of CH 4 and the conversions of both CO 2 and CH 4 were enhanced with increasing reaction temperature, indicating the occurrence of side reactions such as the reverse water gas shi reaction (CO 2 + H 2 / CO + H 2 O).7,29In the absence of CO 2 feeding (pure CH 4 ), a signicant amount of CO was formed over different catalysts due to the decomposition of methane (CH 4 / C + H 2 ) and a further oxidative regeneration (C + O 2 / CO x ) process.54The H 2 /CO ratios were also increased as the reaction temperature increased because the RWGS reaction would gradually be prevented at elevated temperatures.46The presence of CO 2 promoted CH 4 conversion (Fig.6, Tables5 and 6), catalyst stability and H 2 /CO ratio as compared to the catalysts in the absence of CO 2 (Fig.S1 †) at all reaction temperatures.The catalytic stability of the catalysts was investigated over 10 h at varied temperatures (550, 650, and 700 °C) and constant mass of catalyst.Fig.S1† depicts the CH 4 , and CO 2 conversions, as well as the H 2 /CO molar ratio as a function of time on-stream (TOS).These results conrmed the better performance of the metal oxide(s)-doped Ni/Al 2 O 3 -CeO 2 catalyst as compared to the Ni/Al 2 O 3 -CeO 2 catalyst.The results revealed that all catalysts had good activity and stability during 10 h on stream (TOS).It may be concluded that the carbon deposition on the surface of the catalyst was minimal, resulting in the stable conversion and
14Compared with FeNi/Al 2 O 3 -CeO 2 and MgFe/Ni/Al 2 O 3 -CeO 2 catalysts, it can be assumed that the highly dispersed and NiO in Pt/FeNi/Al 2 O 3 -CeO 2 and Pt/MgFe/Ni/Al 2 O 3 -CeO 2 could be more easily reduced by hydrogen atoms from the spillover effect due to the unique interactions between Ni, and Pt species and served as metallic nuclei to facilitate the reduction of Fe 2+ and Ni 2+ .The Al 2 O 3 -CeO 2 support plays a key role in the active site dispersion, activity and stability.To improve the reducibility, and enhance the oxygen mobility and metal dispersion, g-Al 2 O 3 material was modied by adding CeO 2 and the formation of the Al 2 O 3 -CeO 2 support.Further addition of metal oxide promoters improves the reducibility and chemisorption capacity of Ni-based Al 2 O 3 -CeO 2 catalysts due to a better dispersion of bi/tri-metallic catalysts on the Al 2 O 3 -CeO 2 support.
43,42 -CeO 2 catalysts have only weak and moderate basic sites.Therefore, the intensity of TPD peaks became higher, indicating the improved CO 2 adsorption capacity.The higher adsorption of acidic CO 2 over the surfaces of the MO x -promoted catalysts conrmed that these catalysts are more basic as compared to Ni-based Al 2 O 3 -CeO 2 .It is well-established that the basic catalysts could improve the adsorption of CO 2 during the DRM reaction, which supplies the surface oxygen to suppress the coke deposition.This nding is in agreement with previous studies.41,42Itcanbeconcluded that the Mg 2+ -containing supports have an increased number of basic sites (greater amount of desorbed CO 2 ) with respect to the support.Notably, Mg-containing zirconias showed more basic sites as compared to Ni/Al 2 O 3 -CeO 2 .Similar results over magnesia-zirconia oxides were reported by Moreno et al.43Moreover, it was observed that Pt/FeNi/Al 2 O 3 -CeO 2 and Pt/MgFe/Ni/Al 2 O 3 -CeO 2 catalysts doped with Pt showed high basicity as compared to FeNi/Al 2 O 3 -CeO 2 and MgFe/Ni/Al 2 O 3 -CeO 2 13, 33129-33145 | 33133 Paper RSC Advances the catalysts promoted with MO x have weak, moderate and strong basic sites, whereas Ni-based Al

Table 2
Summary of CO 2 -TPD of Ni-based Al 2 O 3 -CeO 2 composite catalysts Catalysts CO 2 desorption a (mmol g −1 ) 60-200 °C 200-400 °C 400-800 °C Total a The amount and strength of base sites were estimated from CO 2 -TPD proles.

Table 3
Results of FTIR spectra of pyridine adsorption 51The amounts were calculated by Emeis.51 −1 ) for Al 2 O 3 -CeO 2, however, it was increased by the addition of Ni and other metal promoters.As shown in Fig. 6A-C, Ni-based Al 2 O 3 -CeO 2 catalysts exhibited almost two-fold the CH 4 and CO 2 conversions as compared the Al 2 O 3 -CeO 2 support in the temperature range investigated.Previous studies have demonstrated that the high metal dispersion led to a large number of active sites and consequently, high activity. 7,29At all reaction temperatures, the Ni-based Al 2 O 3 -CeO 2 catalyst performance was further increased signicantly and activity follows the trend Pt/MgFe > MgFe > PtFe > FeNi for both CH 4 and CO 2 .The doping of Fe 2 O 3 into the Ni-based Al 2 O 3 -CeO 2 catalyst resulted in an almost two-fold increase for both the CH 4 and CO 2 conversions (Fig. 6A-C, Tables 60,56,58cellent catalytic activity and long catalytic stability were observed over a MgFe/Ni/Al 2 O 3 -CeO 2 catalyst.As shown in the FTIR characterization, Mg doping enhanced the Lewis basicity, which is in favor of the chemisorption of CO 2 that would accelerate the reaction, CO 2 + C = 2CO, thus inhibiting the carbon deposition.50,56,58ThePt-dopedMgFe/Ni/Al 2 O 3 -CeO 2 catalyst showed the best performance for H 2 selectivity; it performed better than the other ve catalysts in the whole process, and reached 0.97 at 650 °C as shown in Fig.6A-C, Tables5 and 6, while both Ni/Al 2 O 3 -CeO 2 and Al 2 O 3 -CeO 2 catalysts showed higher CO selectivity at 550 °C.The Pt/ Mg-Fe/Ni/Al 2 O 3 -CeO 2 showed the highest CH 4 and CO 2 conversions and H 2 /CO values for all the temperatures, which indicated that the cooperative interaction between metals and the support could suppress the RWGS reaction.Therefore, Pt/ MgFe/Ni/Al 2 O 3 -CeO 2 is considered a promising candidate for the DRM reaction in terms of activity, stability and selectivity.Furthermore, as summarized in Fig.6A-C, Tables5 and 6, the highest specic activity was obtained, which was followed by Pt/ MgFe/Ni/Al 2 O 3 -CeO 2 > MgFe/Ni/Al 2 O 3 -CeO 2 > PtFe/Ni/Al 2 O 3 -CeO 2 > Fe/Ni/Al 2 O 3 -CeO 2 .Surface basicity, oxygen vacancy and redox properties are crucial for enhancing the CO 2 adsorption capacity and carbonate species formation.59Theaddition of metal oxide(s) and rare earth elements (Ce 2 O 3 ) effectively enhance the surface basicity and redox properties of the catalysts, which further affect the CO 2 adsorption capacity of the Al 2 O 3 -CeO 2 catalyst.The characterization analyses and catalytic tests revealed that the introduction of metal oxide(s) into the Ni/ Al 2 O 3 -CeO 2 catalyst generated more coordination unsaturated Ni atoms, oxygen vacancies, defects and active sites for the DRM reaction.The existence of Pt can initiate the NiO reduction process by the rapid dissociation of H 2 and migration of atomic H to the NiO surface by the hydrogen spillover phenomenon, which can retrain Ni in the metallic state under DRM conditions.Niu et al.60demonstrated that the metal with lower electronegativity enhanced CO 2 activation, which had a positive impact on the surface oxygen concentration and promoted the oxidation of the surface carbon species, thus reducing the carbon formation and improving the catalyst stability.The addition of Pt to the catalyst stabilized the size of the Ni particles and prevented Ni from becoming encapsulated in carbon.This phenomenon occurs because of nickel particles being

Table 4
61ysical properties of the investigated samples obtained from the nitrogen physisorption of Ni-based Al 2 O 3 -CeO 2 composite catalysts Estimated by the Brunauer-Emmett-Teller (BET) at the p/p o in the range of 0.05-0.30.b Micropore area and micropore volumes were determined using the t-plot method.cEstimatedbyBJH at the adsorbed amount at the p/p o = 0.99 single point.dEstimatedbya t-plot.eEstimatedbyBJH desorption average pore diameter.anchoredbyseveralcarbon layers that grow in multiple directions.According to García-Diéguez et al.,61Pt addition to the Ni/ Al 2 O 3 catalyst decreases global carbon synthesis, and the C generated in tri-metallic catalysts (Pt/M-Ni/Al 2 O 3 -CeO 2 ) appears to be less tightly attached to the Ni than in the monometallic and bi-metallic Ni/Al 2 O 3 -CeO 2 catalyst.Furthermore, unlike Ni, carbon is gasied on Pt particles rather than dispersed, resulting in the constant activity of Pt sites.According to Niu et al., the electronic structure of active sites is modied in bimetallic catalysts, which affects the adsorption of specic reagents. a

Table 6
Catalytic activity from DRM over various catalysts a Al 2 O 3 CeO 2 (Al/Ce = 50/50) 700 °C, CH 4 /CO 2 = 50/50, 12 000 mL g −1 h −1 , 1 bar Al 2 O 3 CeO 2 (Al/Ce = 50/50) 700 °C, CH 4 /CO 2 = 50/50, 12 000 mL g −1 h −1 , 1 bar On the surface of catalysts, this will encourage the dissociation of CO 2 into CO and O.The oxidation of surface carbon and CH x , which affects the product's H 2 /CO ratio and the rate of coke generation, is another process that is greatly aided by the presence of O*.Tri/bimetallic Pt-modied Ni-based Al 2 O 3 -CeO 3 catalyst clusters improve catalytic activity, lower the energy barrier for CO 2 activation, and encourage the creation of surface O* via CO 2 adsorptive dissociation while lowering the activation energy for CH 4 dissociation.Surface oxygen density may be increased by the tri/bimetallic Pt-modied Ni-based Al 2 O 3 -CeO 3 catalysts a n.a.(not available).© 2023 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2023, 13, 33129-33145 | 33139 , plausible catalytic mechanism) and the CO disproportionation reaction (step(19), plausible catalytic mechanism) in the methane dry reforming reaction may cover the metal active sites and cause the catalyst to deactivate quickly.A 10 hours longterm durability test at 700 °C was carried out to further evaluate the potential utilization of M-Ni/Al 2 O 3 -CeO 2 catalysts.The small decrease in catalytic activity seen in Fig.S1† and Table7could be attributed to a change in catalyst surface during the reaction process.The M-Ni/Al 2 O 3 -CeO 2 catalysts exhibited strong carbon resistance over a 10 hours procedure.Fig.S1† and Table6indicate that the M-Ni/Al 2 O 3 -CeO 2 catalysts maintained their CH 4 conversion, CO 2 conversion, and H 2 /CO ratio in the presence of minimal carbon deposition.This is due to the great dispersion of Ni on the support, which prevented Ni active site agglomeration and sintering and the strong metal-support contact, which kept the Ni nanoparticle size stable during high-temperature reactions.This is a well-established H 2 -TPR prole.
71d Table2).According to our ndings, the addition of metal oxides can effectively increase the surface basicity of the catalysts, thus improving the CO 2 adsorption capacity of metal oxides (M = Pt, Mg, and Fe)doped Ni/Al 2 O 3 -CeO 3 catalysts.This is consistent with the in situ DRIFTs analysis via Dengsong et al.71The adsorbed CO 2 can form two kinds of carbonate species on the catalyst surface: bidentate carbonates and monodentate carbonates.Meanwhile, the active intermediate CH x can react with these generated carbonates; bidentate carbonates, in particular, are more suitable for CH x conversion.Furthermore, the produced carbonates can efficiently react with the deposited carbon; therefore the presence of the carbonates may assist in the removal of the deposited carbon, resulting in increased catalytic stability.The redox property and its effect on oxygen vacancy are crucial for improving the catalytic performance.Due to the coexistence of redox pairs, the Pt, Mg, Fe, and Ce-modied catalysts showed enhanced redox properties, thereby resulting in abundant oxygen vacancies among the M-Ni/Al 2 O 3 -CeO 3 catalysts as shown in the H 2 -TPR proles (Fig.2).The abundant oxygen vacancies can provide additional active oxygen and more active sites for CO 2 and CH 4 activation.Active oxygen species played an essential role in reducing carbon deposition.Because surface active oxygen species can react with deposited carbon, this helps to prevent catalyst deactivation during the DRM process.Furthermore, the improved redox characteristics can enable electron transport, which can increase the rate of CH 4 / CO 2 conversion as well as the elimination of deposited carbon.

Table 7
Summary of the weight loss of Ni-based Al 2 O 3 -CeO 2 composite catalysts Spent catalyst in the absence of CO 2 .b Spent catalyst in the presence of CO 2 . a CO * 2 þ O LTÀ1 4O LT þ CO*; ðNi-MO x sitesÞvacancy filling(20)H 2 + O x 4 O LT−1 + H 2 O, (Ni-MO x sites) COOH* + * 4 CO* + OH*, (Ni-MO x sites) (13) HCO* + * 4 +CO* + H*, (Ni-MO x sites)(14)CH * x þ OH*4CH x OH* þ *; ðNi-MO x sitesÞ(15)CH x OH* + (x + 1)* 4 CO* + (x + 1)H*, (Ni-MO x sites) (16)CH * x þ O*4CH x O*; oxidation of adsorbed CH x ðNi-M sitesÞ(17)CH x O* + x* 4 CO* + xH*, (Ni-MO x sites)(18)C* + O LT 4 CO + O LT + *, (Ni-MO x sites)(19)CO* 4 CO, (Ni-MO x sites) Pt, Fe, and Mg were doped into Ni-based Al 2 O 3 -CeO 2 composite catalysts via an impregnation method and evaluated for DRM reaction.All doped Ni-based Al 2 O 3 -CeO 2 composite catalysts presented signicantly higher activity and H 2 selectivity as compared to the Ni-based Al 2 O 3 -CeO 2 composite catalysts.Characterization results showed that compared to unmodied catalysts, the catalysts treated with metal oxide (Mg, Fe, and Pt) particles had a signicant number of basic sites for enhancing CO 2 adsorption.The most stable and efficient conversion of CO 2 and CH 4 was achieved by the modied catalysts because they displayed the highest Ni particle dispersion, which can effectively stop the migration of Ni species.Additionally, as shown by the H 2 -TPR prole, the co-doped catalysts displayed high levels of redox cycling, which enhanced the generated oxygen vacancies and helped eliminate carbon deposition.Although all metal-doped Ni-based Al 2 O 3 -CeO 2 catalysts showed slightly lower surface areas than the Ni/Al 2 O 3 -CeO 2 catalyst and Al 2 O 3 -CeO 2 support, they presented signicantly higher activity and H 2 selectivity compared to Ni-based Al 2 O 3 -CeO 2 composite catalysts.Catalytic activity results revealed the impact of secondary and tertiary metal doping on the Ni-based Al 2 O 3 -CeO 2 catalyst and the highest methane (>85%) and CO 2 (∼90%) conversions and high selectivity towards H 2 /CO ratio (0.97) were obtained over the Pt/MgFe/Ni/Al 2 O 3 -CeO 2 composite catalyst.The order of activity of the catalysts, based on the turnover frequencies, was Pt/MgFe/Ni/Al 2 O 3 -CeO 2 > MgFe/Ni/Al 2 O 3 -CeO 2 > Pt/FeNi/Al 2 O 3 -CeO 2 > FeNi/Al 2 O 3 -CeO 2 > Ni/Al 2 O 3 -CeO 2 .The observed better Pt/MgFe/Ni/Al 2 O 3 -CeO 2 stability could be due to favorable changes in the distribution of surface basic sites, and better Ni dispersion.